This invention relates to oil and gas well casing hanger apparatus and in particular to a casing hanger assembly for supporting the casing at the mudline of a subsea well.
In the drilling of oil and gas wells concentric casing strings are hung and cemented in place as the drilling progresses to increasing depths. When drilling a subsea well from a fixed platform it is desirable to support the casing weights from the mudline with a blowout preventer located at the platform. Risers extend from the blowout preventer to the support location of substantially the same size as the casing string itself.
In supporting additional casing within the previously-run string, a limited annular space is available for this support. Furthermore, the support must be arranged in such a was as to permit flow through the annular space to facilitate cementing operations.
It is known to run the new casing hanger with its string of casing with a diametrically compressible collet around it being urged outwardly. The collet includes specially-shaped support shoulders extending outwardly which engage grooves in the previously-set hanger body. The new casing hanger body then rests on this collet.
Means such as shear pins are required to carry the collet on the hanger body at least until it enters the casing below the BOP and sometimes to pull the collet down until it reaches the support elevation. Other systems use the load support shoulder to push the collet down after means are provided to constrain the collet until it enters the casing string.
As wells approach greater depths, increasing load must be carried through these hangers; and effective utilization of the available space and material with structures having minimum stress concentrations is desirable. The load supporting segments which enter the previously-run hanger body should be fully engaged despite any mud that may have previously accumulated therein. Furthermore, the load shoulder between the collet and the newly-run casing hanger body is preferably shaped to provide the most desirable stress distribution in the hanger body.
It is also desirable where possible to avoid shear mechanisms which will leave loose metal particles that end up falling down hole or become lodged in the mechanism.